Summary

Switch-over control is the ability to use 2 sensor inputs into one instrument. The first sensor is used for one set of conditions (such as low temperature). The second sensor is used for the other set of conditions (such as high temperature). Between the two customer defined ranges, Eurotherm uses a ramped blending of the two sensors in order to have a seamless transition between the sensors.

Applications include: thermocouple vs. IR pyrometer applications, atmosphere temperature vs. load temperature applications, or long lag applications where one sensor is used during start-up and a different sensor (or sensor location) is used during the production mode.

Introduction

Switch-Over control is available in the 2408 and 2404. For example, it is implemented on high-temperature furnaces that require high accuracy at both low and high temperatures. It is usually difficult to find one sensor that is accurate at both high and low temperatures. Typically the low temperature sensor will work accurately over the ranges of 100°F to 1000°F. The second sensor will work accurately from 1000°F to 2200°F.

Switch-Over control allows smooth transfer between a thermocouple (low temperature) and a pyrometer (high temperature). The sensor that works in the lower range of measured values must be connected to input 1, and that sensor working on the higher range to input 2. The minimum value for input 2 can be less than or greater than the maximum for input 1. It is recommended that there be a region common to both sensors of at least 10% of the full scale. When the measured value is below this region, the 2400 will control using input 1. When the measured value is above this region, the 2400 will control using input 2. In the common region, the Eurotherm 2400 instrument controls using a measured value that is a proportional blend of the two values. The equation used to calculate the measured value in the common region is as follows: X=X(I/P1) + (1-X)I/P2 (X equals the measured value)

X=(I/P1 Max - Previous value for X) divided by (I/P1 Max - I/P2 Min)

A full-scale-high alarm can be used to control the removal of the thermocouple from the load once the transfer to the high temperature region is complete.

Features

* The main process variable input 1 (IP.1), should always be configured as the lower temperature range.

* If the thermocouple is removed from the load after the transfer to input 2, the algorithm cannot transfer back to input 1.

* If either input breaks, sensor break will be detected.

Instrument Setup

The FUNC parameter in the configuration must be configured as trAn (transfer). The IP list in the configuration is used to set up low temperature input (IP.1) and the 3A list in the configuration is used to set up the high temperature input (IP.2).

Configuration Setup

Switch-Over control is available in the Eurotherm model 2408 (1/8 din size) or 2404 (1/4 din size) instrument. (Software version 3.x or greater).

The 2408 or 2404 instrument can also be used in other 2 sensor applications. These include:

* control based upon max sensor (or min, or X% of #1 and Y% of #2, or average)

* control based upon sensor #1 until sensor #2 is selected. This selection occurs due to a digital input (push button, limit switch, motor started condition, etc.), a signal from a PC (via EIA 232 or EIA 485), or a signal from a PLC®

* control based upon sensor #1 and an alarm point based upon sensor #2